1. Field of the Invention
The present invention relates to an image processing technique and, more particularly, to an edge emphasis technique of image data based on the image printing characteristic.
2. Description of the Related Art
A copying machine which optically scans a document image and prints a scanned image on a print medium such as a paper sheet or the like is known. In general, the edge of an image optically scanned by a scanning unit (scanner) of a copying machine is smoother than that of a document image. For this reason, an image with poor sharpness is output when the scanned image is printed on a print medium intact.
Hence, Japanese Patent No. 2620368 (reference 1) discloses an image signal processing method with the following characteristic feature. That is, in this method, when the distribution pattern of an image signal at a point of interest is convex upward, a signal replaced by an image emphasis signal larger than an image signal at the point of interest is output; when the distribution pattern is convex downward, a signal replaced by an image emphasis signal smaller than the image signal at the point of interest is output; otherwise, a signal replaced by an original image signal or unsharp signal is output. Also, Japanese Patent Laid-Open No. 07-288768 (reference 2) discloses a video signal processing method which replaces the level of a pixel of interest of an image by a replacement level generated using the pixel of interest and pixels which neighbor the pixel of interest. Furthermore, Japanese Patent Laid-Open No. 2004-056252 (reference 3) discloses a method which avoids not only poor sharpness described above but also troubles such as dot omission and disappearance of thin lines at a reduction scale by changing the resolution of a data input device or output device.
However, upon applying edge emphasis by the aforementioned method, the output image quality the user desired cannot always be improved. FIG. 8 illustrates a state in which edge emphasis processing is applied to image data including a bold character. Reference numeral 801 denotes an unprocessed image immediately after a document image is scanned; and 802, an image upon printing an image that has undergone the edge emphasis processing on a print medium. As can be seen from FIG. 8, application of the edge emphasis processing results in an illegible character since the spacings of black parts of the character are narrowed down.
This problem arises from the difference of an expression region of a pixel as a unit of image processing between the image processing such as edge emphasis and the like and an output device including a print process that reproduces the image. In particular, this problem occurs prominently when a bold character as the type of character is scanned or when a document image includes a complicated kanji character (that with a large number of strokes). Also, an ink-jet printer causes ink bleeding on a print medium, and the problem of illegibility due to character collapse appears conspicuously. Since the problem of ink bleeding depends on the characteristics of inks and print media, it varies depending on different inks and print media or their combinations.
For this reason, the techniques disclosed in references 1 and 2 above cannot solve these problems. When both the input and output devices have a sufficiently high degree of freedom in settings in association with the resolution, the technique disclosed in reference 2 can eliminate the aforementioned problems. However, the device cost increases considerably.